It is known that solid organic waste material may be treated under either anaerobic or aerobic conditions to produce a bioactive, stable end product that, for example, may be used as compost for gardens. This process is achieved through the action of, respectively, anaerobic or aerobic microorganisms that are able to metabolise the organic waste material to produce the bioactive, stable end product.
It is also known that the aerobic decomposition of solid organic waste material takes place in the presence of oxygen. The temperature of the waste material rises as the energy produced during aerobic decomposition is released as heat, often reaching temperatures of approximately 75° C. under ambient conditions. The solid end product is often rich in nitrates which are a readily bio-available source of nitrogen for plants, making the end product particularly suitable as a fertiliser.
It is further known that the anaerobic digestion of solid organic waste material takes place in, the absence of oxygen. Anaerobic microbial metabolism is understood to be optimised when the organic material is heated to temperatures at which mesophilic or thermophilic bacteria are operative. The process of anaerobic microbial metabolism results in the production of biogas, in turn predominantly methane and carbon dioxide. The solid product of the process is often rich in ammonium salts. Such ammonium salts are not readily bio-available and are, consequently, generally treated under conditions in which aerobic decomposition will occur. In this manner the material is used to produce a product that is bio-available.
Typically, systems for the biodegradation of organic waste material are directed to either aerobic or anaerobic processes. However, there are a small number of systems that have sought to combine both anaerobic and aerobic biodegradation processes. The processes of German Patent 4440750 and International Patent Application PCT/DE1994/000440 (WO 1994/024071) each describe the combination of an anaerobic fermentation unit and an aerobic composting unit. Importantly, these systems describe discrete and separate vessels for the aerobic and anaerobic biodegradation processes.
International Patent Application PCT/AU00/00865 (WO 01/05729) describes an improved process and apparatus in which many of the inefficiencies of the previous processes and apparatus are overcome. The improved process and apparatus are characterised at a fundamental level by the sequential treatment of organic waste material in a single vessel, through an initial aerobic step to raise the temperature of the organic waste material, an anaerobic digestion step and a subsequent aerobic treatment step. During the anaerobic digestion step a process water or inoculum containing micro organisms is introduced to the vessel to create conditions suitable for efficient anaerobic digestion of the contents and the production of biogas. The introduced inoculum also aids in heat and mass transfer as well as providing buffer capacity to protect against acidification. Subsequently, air is introduced to the residues in the vessel to create conditions for aerobic degradation. It is further described that the water introduced during anaerobic digestion may be sourced from an interconnected vessel that has undergone anaerobic digestion.
Given the relative size of the vessel, the OFMSW is subjected to relatively high and varied consolidation pressures, which results in a diminished, or reduced, capacity for the liquid introduced during the anaerobic digestion to penetrate all parts of the material within the vessel. As such, there is a reduced biogas yield from the OFMSW. Therefore, it is desirable to recirculate the OFMSW during anaerobic digestion in order to improve liquid penetration into the OFMSW. This in turn is expected to lead to greater biogas production.
Additionally, the sequential treatment of the OFMSW in the single vessel through anaerobic digestion and aerobic, composting stages brings with it certain challenges, resulting in part from the fundamentally ‘batch’ nature of the process. That is, compared to other prior art processes in which the OFMSW is simply transferred to another reactor between stages care is needed to ensure that conditions are transitioned through to those most suitable for each stage when the other stage has finished. This requires that a certain amount of time be taken in these transitions. One such problematic transition is the change from anaerobic digestion to aerobic composting. In order for this change to occur as quickly as possible it is necessary for the dewatering of the solids.
Finally, it is also desirable to maintain a certain moisture content after the anaerobic digestion to aerobic composting transition. It is further desirable to achieve this transition without having to open the reactor vessel to atmosphere. The present invention has as one object thereof to overcome substantially the abovementioned problems of the prior art, or to at least provide a useful alternative thereto.
Korean Patent 10-0722407 to Sib Co., Ltd discloses a high pressure and temperature aerobic digester intended for the treatment of high strength waste water, including livestock excreta. The specification describes an air-lift reactor that utilises downward liquid flow through a central column or “circulation induction pipe”. The elevated pressure is said to enhance oxygen transfer and dissolution, and the metabolism of the high temperature aerobic microorganisms present is said to heat the recirculated liquid. This is further said to process the “high concentration organic waste water” within a time frame of 3 to 5 days. Specifically, it is the water or liquid that is recirculated within the aerobic digester vessel. This occurs by way of forced convection or “air lift”. The digester described in this Korean patent does not provide a solution to the recirculation of solid materials under pressure.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.